Isabella Rossellini and Kunal Nayyar also appear in Johan Renck’s film of the Jaroslav Kalfur novel ‘Spaceman of Bohemia,’ featuring Paul Dano as the voice of an alien arachnid.

The first thing to jump out at you about Spaceman is how unusually restrained Adam Sandler is in the title role of Czech astronaut Jakub. The actor’s goofy sweetness is entirely subsumed into a 21st century Major Tom, a character steeped in the profound sadness and insecurity of isolation, who responds to the arrival of an alien intruder aboard his ship — either dating back to the birth of the universe or simply manifested by Jakub’s anxious mind — like an addled neurotic who’s found a really good therapist.

While the Netflix feature, adapted by Colby Day from Jaroslav Kalfar’s novel Spaceman of Bohemia, has its share of the usual narrative glitches — communication system failures, corporate meddling, life-threatening turbulence — it’s primarily a story of marital discord and the wakeup call and atonement of a man too fixated on his dreams to see what’s most precious to him. That, in a nutshell, is the limitation of this engrossing but under-powered sci-fi drama.

Day’s adaptation strips away most of the political background of the 2017 novel, leaving only residual traces of Jakub’s search for redemption for the sins of his father, an informant for the Communist Party publicly disgraced following the fall of the regime. That thread isn’t sufficiently robust here to persuade the viewer that Jakub’s heroic goal is about much more than personal glory. It’s a testament to the frazzled warmth of Sandler’s performance that even when his vision is most blinkered, the character remains sympathetic.

Four years after a cloud of particles from deep space cast a glittering purple glow over Earth’s night sky, the Czech government has been the first to step up and launch an investigatory mission, with South Korea nipping at their heels. Jakub is six months into that solo voyage and 500 million kilometers from home when cabin fever starts getting to him. He’s not sleeping, the toilet keeps malfunctioning, and most worrying of all, he hasn’t heard from Lenka in some time.

Both the head of the Czech space program, Commissioner Tuma (Isabella Rossellini), and Jakub’s main contact back at the base, Peter (Kunal Nayyar), focus chiefly on getting their man to the outer limits of Jupiter to collect particle specimens while keeping the sponsors happy. In an amusing moment early on, a stressed Jakub rattles off the text of an AntiQuease commercial — the mission’s official nausea medication — during a public broadcast in which he’s required to be gung ho about how it’s going, keeping his concerns to himself.

That’s where the astronaut’s alien surrogate shrink comes in. In what appears to be a dream, a creepy-crawly form is seen moving under the skin of Jakub’s face, its pedipalps first making an abortive attempt to exit via his nostrils before a sizeable spider emerges from his mouth. Soon after, he encounters the now giant six-eyed arachnid lurking around an airlock. Unfazed by Jakub blasting it with decontaminant, the intruder explains that it wishes him no harm, only to lessen his solitude.

Jakub resists the creature’s overtures for a time, keeping his distance and agonizing over what he worries is a mental breakdown. But gradually, the spider becomes such a soothing presence that he gives it a name, Hanuš. There’s much talk about the fate of Hanuš’ planet and the origins of the universe in the purple cloud, its energy inexorably pulling them in. But all that outer-space expansiveness ends up being mere background to Jakub’s examination of his marital failings.

As shown in his more serious roles in films like Punch-Drunk Love, The Meyerowitz Stories and Uncut Gems, Sandler has more range than he’s often called upon to use, and Mulligan is always watchable, even in a one-note role.